1. Field of Endeavor
The present invention relates to the technology of gas turbines, and more specifically to a gas turbine of the axial flow type.
2. Brief Description of the Related Art
A gas turbine is composed of a stator and a rotor. The stator constitutes a casing with stator heat shields and vanes installed in it. The turbine rotor, arranged coaxially within the stator casing, includes a rotating shaft with axial slots of fir-tree type used to install blades. Several blade rows and rotor heat shields are installed therein, alternating. Hot gas formed in a combustion chamber passes through profiled channels between the vanes, and, when striking against the blades, makes the turbine rotor rotate.
For the gas turbine to operate with a sufficient efficiency it is essential to work with a very high hot gas temperature. Accordingly, the components of the hot gas channel, especially the blades, vanes and heat shields, of the turbine experience a very high thermal load. Furthermore, the blades are at the same time subject to a very high mechanical stress caused by the centrifugal forces at high rotational speeds of the rotor.
Therefore, it is of essential importance to cool the thermally loaded components of the hot gas channel of the gas turbine.
In the prior art, it has been proposed to provide channels for a blade cooling medium within the rotor shaft itself (see for example EP 909 878 A2 or EP 1 098 067 A2 or U.S. Pat. No. 6,860,110 B2). However, such a cooling configuration requires the complex and costly machining of the rotor or rotor disks.
A different cooling scheme according to the prior art is shown in FIG. 1. The gas turbine 10 of FIG. 1 includes a plurality of stages the first three of which are shown in the Figure. The gas turbine 10 includes a rotor 13, which rotates around a central machine axis, not shown. The rotor 13 has a rotor shaft 15 with axial slots of the fir-tree type used to install a plurality of blades B1, B2 and B3. The blades B1, B2 and B3 of FIG. 1 are arranged in three blade rows. Interposed between adjacent blade rows are rotor heat shields R1 and R2. The blades B1, B2, B3 and the rotor heat shields are evenly distributed around the circumference of the rotor shaft 15. Each of the blades B1, B2 and B3 has an inner platform, which—together with the respective platforms of the other blades of the same row—constitutes a closed ring around the machine axis.
The inner platforms of blades B1, B2 and B3 in combination with the rotor heat shields R1 and R2 form an inner outline of the turbine flow path or hot gas path 12. At the outer side, the hot gas path 12 is bordered by the surrounding stator 11 with its stator heat shields 51, S2 and S3 and vanes V1, V2 and V3. The inner outline separates a rotor cooling air transit cavity, which conducts a main flow of cooling air 17, from the hot gas flow within the hot gas path 12. To improve tightness of the cooling air flow path, sealing plates 19 are installed between adjacent blades B1-B3 and rotor heat shields R1, R2.
As can be seen from FIG. 1, air cools the rotor shaft 15 when flowing in the axial direction along the common flow path between blade necks of blades B1-B3 and rotor heat shields R1, R2; this air passes consecutively through the inner cavity of the blade B1, then in turn through blade B2 and blade B3 cavities.
However, blades contained in modern turbines operate under heavier conditions than vanes because they are, in addition to the effects of high temperatures and gas forces, subject to loads caused by centrifugal forces. To create an efficient blade having large life span, one should solve an intricate complex technical problem.
To solve this problem successfully, one should know the cooling air pressure at the blade inner cavity inlet as precisely as possible. Therefore a serious shortcoming of the rotor design presented in FIG. 1 is that the cooling air pressure loss increases in an unpredictable way as this air passes from the first stage blade B1 to the third stage blade B3. This is caused by air leakages into the turbine flow path 12 through slits between adjacent blades and rotor heat shields. This disadvantage prevents effectively cooled blades from being designed since total cross section area of the above-mentioned slits depends on a scatter of part manufacturing tolerances and on doubtful effectiveness of sealing plates 19.